Go to Calendar.
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Select the Calendar tool from the Tool Menu in your site.
Choose Add/Edit Fields.
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To add a new field:
Enter the Field Name into the text box provided.
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Select the Create Field button.
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The new field will now display in the list of existing fields on this page. It will also show up as a field for each calendar event when you add or edit an event.
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To delete an existing field:
Select the check box next to the field or fields you would like to remove.
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Select the Remove selected fields button.
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The removed field will no longer display in this list, or on the calendar event details screen.